The increasing use of portable electronics has driven research in the area of portable electric generators. Thermoelectric (TE) power sources have been found to be especially useful. TE power sources typically comprise three parts: a heat source, a heat sink, and a thermopile. The thermopile, consisting of a number of thermocouples connected in series, serves to convert some of the thermal energy into electrical energy. TE power sources generate electric power based on creating a thermal gradient across the thermocouples of the thermopile. The TE power source operates to convert the thermal energy to electric power by accepting thermal energy on a “hot” side or junction, passing it through the thermopile and rejecting heat to a “cold” side or junction.
Certain TE power sources and TE thermocouples in particular are formed using semiconductor materials. Semiconductor materials with dissimilar characteristics are connected electrically in series (to form thermocouples) and thermally in parallel, so that two junctions are created. The semiconductor materials are typically n-type and p-type. In a typical thermoelectric device, the electrically conductive connection is formed between the p-type and n-type semiconductor materials. These materials are so named because of their structure: the n-type has more electrons than necessary to complete a perfect molecular lattice structure while the p-type does not have enough electrons to complete a lattice structure. The extra electrons in the n-type material and the holes left in the p-type material are called “carriers.” The carriers are driven from the hot junction to the cold junction as a result of thermal diffusion resulting in an electrical current. For thermoelectric cooling, the electrons and holes transport heat as a result of imposed electrical current. Prior art FIG. 1a illustrates a form of such power conversion. Cooling action results from reversing the process.
A semiconductor TE device's performance is limited by the non-dimensional thermoelectric figure of merit (ZT) of the material, where T is the absolute temperature and Z is the thermoelectric figure of merit, Z=sa2/k (a-thermoelectric power, s-electrical conductivity, k-thermal conductivity). Typically TE devices are preferably formed of TE materials having relatively high thermoelectric figures of merit. In certain devices, however, the key objective is to produce power at voltages above 1.0 V in as small or compact a device as possible. The known TE materials having relatively high thermoelectric figures of merit cannot be deposited as thin films on substrates useful for forming small TE power source devices. Thus, although more efficient materials (i.e., materials with high ZT values) are typically better, for many applications it is more important that the resulting device be formed on a flexible substrate. As a result, although there may be some sacrifice of ZT value, using a TE material depositable on a substrate that allows fabrication of a small device with a relatively high voltage (without the need for a dc-dc converter) is better for certain applications. Unfortunately no such materials and methods are yet available.
Devices having ZT values of greater than 2.0 have been reported for Bi—Te/Sb—Te superlattices grown on single crystal GaAs. Such devices are not, however, suitable for many applications where hundreds or thousands of elements must be placed in a relatively small package.
Despite the potential and promise of TE devices, existing TE power sources have limited efficiency and electric potential when relatively small devices are made. Conventional semiconductor deposition techniques for making TE devices, such as electrochemical deposition, are not well suited for building optimally designed TE power sources. Difficult syntheses have limited the construction of many TE devices to bulk materials or minute quantities—each suffering from shortcomings in size or performance.
For example, currently available TE modules have structures similar to that depicted in prior art FIG. 1b, with each distinct thermoelement typically having a length and width on the order of a few millimeters. Such modules are described, for example, in U.S. Pat. No. 6,388,185 and C. B. Vining, Nature 413:577 (Oct. 11, 2001). These modules cannot provide voltages that readily match the input requirements of many devices, including power conditioning electronics.
A practical approach to building high-voltage, thin-film TE devices capable of microwatt power output in relatively small packages is needed. In addition, TE devices using a temperature gradient of about 10° C. or less would be helpful as well as TE devices operating at or near ambient temperatures. A number of applications require TE devices that operate at such temperatures and/or on such temperature gradients. For example, sensors used for building climate control or for other applications such as military applications where ambient energy is utilized if possible, operate on only 5 to 20° C. temperature differences.